page_title: Britannia Bridge
section_title: Construction and use
hierarchical_section_title: Britannia Bridge / Construction and use
context_page_description: Britannia Bridge is a bridge across the Menai Strait between the island of Anglesey and the mainland of Wales. It was originally designed and built by the noted railway engineer Robert Stephenson as a tubular bridge of wrought iron rectangular box-section spans for carrying rail traffic. Its importance was to form a critical link of the Chester and Holyhead Railway's route, enabling trains to directly travel between London and the port of Holyhead, thus facilitating a sea link to Dublin, Ireland.
  Decades prior to the building of the Britannia Bridge, the Menai Suspension Bridge had been completed, but this structure carried a road rather than track; there was no rail connection to Anglesey prior to its construction. After many years of deliberation and proposals, on 30 June 1845, a Parliamentary Bill covering the construction of the Britannia Bridge received royal assent. At the Admiralty's insistence, the bridge elements were required to be relatively high in order to permit the passage of a fully rigged man-of-war. In order to meet the diverse requirements, Stephenson, the project's chief engineer, performed in-depth studies on the concept of tubular bridges.
context_section_description: On 10 April 1846, the foundation stone for the Britannia Bridge was laid, marking the official commencement of construction work at the site. The resident engineer for the structure's construction was civil engineer Edwin Clark, who had previously aided Stephenson in performing the complex structural stress calculations involved in its design process. The first major elements of the structure to be built were the side tubes, this work was performed in situ, using wooden platforms to support it. The construction method used for the iron tubes was derived from contemporary shipbuilding practices, being composed of riveted wrought iron plates ⁵⁄₈ inch (16 mm) thick, complete with sheeted sides and cellular roofs and bases. The same technique as used for the Britannia Bridge was also used on the smaller Conwy Railway Bridge, which was built around the same time. On 10 August 1847, the first rivet was driven.
  Working in parallel to the onsite construction process, the two central tube sections, which weighed 1,800 long tons (1,830 tonnes) apiece, were separately built on the nearby Caernarfon shoreline. Once they had been fully assembled, each of the central tubes was floated, one at a time, into the causeway and directly below the structure. The in-place sections were gradually raised into place using powerful hydraulic cylinders; they were only raised by a few inches at a time, after which supports would be built underneath the section to keep it in place. This aspect of the bridge's construction was novel at the time. Reportedly, the innovative process had been responsible for costing Stephenson several nights of sleep at one stage of the project. The work did not go smoothly; at one point, one of the tube allegedly came close to being swept out to sea before being recaptured and finally pushed back into place. The tubes were manoeuvred into place between June 1849 and February 1850.
  Once in place, the separate lengths of tube were joined to form parallel prestressed continuous structures, each one possessing a length of 1,511 feet (460.6 m) and weighing 5,270 long tons (5,350 tonnes). The pre-stressing process had increased the structure's loadbearing capacity and reduced deflection. The tubes had a width of 15 feet (4.5 m) and differed between 23 feet (7 m) and 30 feet (9.1 m) in overall depth, while also having a 10 foot (3 m) gap between them; they were supported on a series of 15-foot-long (4.6 m) cast iron beams that were embedded in the stonework of the towers. To better protect the iron from the weather, an arched timber roof was constructed to cover both tubes; it was roughly 39 feet (12 m) wide, continuous over their whole length, and covered with tarred hessian. A 12 foot (3.7 m) wide central walkway was present above the roof for the purpose of producing maintenance access.
  On 5 March 1850, Stephenson himself fitted the last rivet of the structure, marking the bridge's official completion. Altogether, the bridge had taken over three years to complete. On 18 March 1850, a single tube was opened to rail traffic. By 21 October of that year, both tubes had been opened to traffic.
  For its time, the Britannia Bridge was a structure of "magnitude and singular novelty", far surpassing in length both contemporary cast beam or plate girder iron bridges. The noted engineer Isambard Kingdom Brunel, a professional rival and personal friend of Stephenson's, was claimed to have remarked to him: "If your bridge succeeds, then mine have all been magnificent failures". On 20 June 1849, Brunel and Stephenson had both looked on as the first of the bridge's tubes was floated out on its pontoons. The construction techniques employed on the Britannia Bridge had obviously influenced Brunel as he later made use of the same method of floating bridge sections during the construction of the Royal Albert Bridge across the River Tamar at Saltash.
  There was originally a railway station located on the east side of the bridge at the entrance to the tunnel, run by the Chester and Holyhead Railway company, which served local rail traffic in both
caption: A section of the original wrought-iron tubular bridge standing beside the modern crossing.
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